The production of large and complex fiber composite structures, such as airplane wings, requires critically dimensioned features to be held within narrow tolerances. Until recently, process for the production of large and complex fiber composite structures did not exist. Large and complex structures had to be broken down into multiple pieces of a more simple design. Each piece was produced individually and then assembled to create the larger structure in a labor-intensive and costly process.
The development of resin film infusion (RFI) methods and the mold tooling used with RFI methods, as demonstrated in U.S. Pat. No. 5,902,535 to Burgess et al., has allowed the production of composite structures up to 70 feet in length. The RFI process involves placing a resin film directly in between, and in contact with, the dry fiber preform and the mold tooling. The mold tooling, resin film and dry fiber preform are then vacuum bagged and inserted into an autoclave. As the temperature and pressure in the autoclave are increased, the resin film melts and is infused through the preform. Large displacements of the mold tooling and fiber perform are not uncommon when thick resin films are used in the RFI process. Large displacements of the mold tooling and the fiber perform can adversely affect the dimensional tolerances of the resulting composite structure.
Vacuum assisted resin transfer molding (VARTM) has also grown over the past few years to include the fabrication of large composite structures for the marine industry, as well as secondary structures for military aircraft, such as engine inlet ducts. In VARTM the liquid resin is infused into the preform by pulling a vacuum on the mold tool. The liquid resin is introduced to the preform with inlet tubes and a manifold system located on the outer surface of the preform. The liquid resin is drawn through the preform via the vacuum pressure. In VARTM, the mold tool requires only slight movement to compensate for bulk reduction in the preform as the vacuum is applied.
Many VARTM structures, however, fail to meet the more stringent structural criteria required for primary structures in aircraft. Primary aircraft structures must have a fiber volume fraction in the range of 57% to 60%. Fiber preforms with thick cross-sections, typical of aircraft primary structures, require significant force application to remove bulk and raise fiber volume fractions to acceptable levels. The fiber volume of thick structures is typically less than 54% due to the relatively low pressure vacuum provided during the VARTM process. In addition, parts made with VARTM are typically relatively simple in design and have a minimal amount of integral structure. The inner mold line is shaped using a soft rubber or nylon vacuum bag, with very little being done to hold the critical dimensions on the mold line surface.
As an alternative approach, U.S. Pat. No. 5,441,692 to Taricco discloses a process for performing VARTM in an autoclave on simple composite structures. The pressure of the inner chamber of the autoclave is reduced concurrent with the reduction of pressure within the VARTM tooling. A reduction of pressure in the autoclave results in zero net pressure across the tooling, significantly reducing the stresses on the tooling. A reduction in the stresses on the tooling allows the tool and cover to be constructed with relatively thin walls, which reduces the weight and thermal capacitance of the tooling. Reducing the weight of the tooling allows for easier handling of the tooling. Also, reduction of the thermal capacitance of the tool reduces the time needed to heat and cool the tool and composite structure.
U.S. Pat. No. 5,015,168 to Boime et al. discloses tooling for use in VARTM that forms a somewhat more complex composite panel that includes a row of panel stiffeners. The tooling includes a block, a sealing bag and calibration parts. The peripheral edge of the bag is connected to the block by a sealing bead and defines a tight volume. The panel is placed in the tight volume between the bag and the block. The calibration parts are placed outside the volume, above the bag and between the panel stiffeners to ensure maintenance of the geometry of the stiffeners during resin transfer. Despite the improvement in structural complexity of the resulting composite part, the use of soft tooling limits the precision to which details can be produced on the part using the process of Biome et al.
Notwithstanding the prior techniques for forming composite structures, it would be advantageous to have a further improved process for the production of complex composite structures to narrow tolerances that have sufficiently high fiber volumes to be used as primary structures on aircraft or marine vessels.